Nanofiber packed beds having enhanced fluid flow characteristics

ABSTRACT

The general area of this invention relates to porous materials made from nanofiber packed beds. More particularly, the invention relates to altering the porosity or packing structure of a nanofiber packed bed structure by blending nanofibers with scaffold particulates having larger dimensions. For example, adding large diameter fibers to a nanotube packed bed to serve as a scaffolding to hold the smaller nanofibers apart and prevent the nanofiber bed structure from collapsing. This increases the average pore size of the mass by changing the pore size distribution and alters the packing structure of the packed bed. The increase in average pore size is caused by the creation of larger channels which improves the flow of liquids or gasses through these materials.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates broadly to nanofiber packed beds having enhancedfluid flow characteristics and to methods of making same and methods ofusing same. More specifically, the invention relates to nanofibers whichare uniformly or non-uniformly blended with supporting scaffoldparticulates to form packed beds having enhanced fluid flow rates and anincreased overall average pore size. Even more specifically, theinvention relates to using such packed beds for a variety of purposesincluding as products such as flow-through electrodes, chromatographicmedia, adsorbant media and filters.

2. Description of the Related Art

Nanofiber mats and assemblages have been previously produced to takeadvantage of the increased surface area per gram achieved usingextremely thin diameter fibers. These prior mats or assemblages areeither in the form of tightly, dense masses of intertwined fibers and/orare limited to microscopic structures (i.e., having a largest dimensionless than 1 micron). Nanofiber mats or assemblages have been previouslyprepared by dispersing nanofibers in aqueous or organic mediums and thenfiltering the nanofibers to form a mat. The mats have also been preparedby forming a gel or paste of carbon fibrils in a fluid, e.g. an organicsolvent such as propane and then heating the gel or paste to atemperature above the critical temperature of the medium, removingsupercritical fluid and finally removing the resultant porous mat orplug from the vessel in which the process has been carried out. See,U.S. Pat. application No. 08/428,496 entitled "Three-DimensionalMacroscopic Assemblages of Randomly Oriented Carbon Fibrils andComposites Containing Same" by Tennent et al. hereby incorporated byreference.

One of the disadvantages of the prior assemblages or mats made by theabove described methods is poor fluid flow characteristics within thestructure. As suspensions of nanofibers are drained of the suspendingfluid, in particular water, the surface tension of the liquid tends topull the nanofibers into a dense packed "mat". Alternatively, thestructure may simply collapse. The pore size of the resulting mat isdetermined by the interfiber spaces which as a result of the compressionof these mats tend to be quite small. As a result, the fluid flowcharacteristics of such mats are poor.

Accordingly, although previous work has shown that nanofibers can beassembled into packed, thin, membrane-like assemblages through whichfluid will pass, the small diameters of the nanofibers results in a verysmall pore structure that imposes a large resistance to fluid flow.

It would be desirable to overcome the above-mentioned disadvantages byproducing a porous packed bed having enhanced fluid flow and an alteredpore size distribution since there are applications for porous nanofiberpacked beds that require fluid passage and the resistance to fluidtransport creates serious limitations and/or drawbacks for suchapplications. The improved fluid flow characteristics brought about bythis invention make such applications more feasible and/or moreefficient.

OBJECTS OF THE INVENTION

It is therefore an object of this invention to provide porous nanofiberpacked bed structures having enhanced fluid flow characteristics and/orincreased average pore size.

It is another object of the invention to provide a composition of matterwhich comprises a three-dimensional, macroscopic nanofiber packed bedmade up of a blend of randomly oriented nanofibers and larger scaffoldparticulates.

It is a further object of the invention to provide processes for thepreparation of and methods of using the nanofiber packed beds havingenhanced fluid flow characteristics.

It is a still further object of the invention to provide improved filtermedia, chromatographic media, adsorbant media, electrodes, EMI shieldingand other compositions of industrial value based on three-dimensionalnanofiber porous packed beds.

The foregoing and other objects and advantages of the invention will beset forth in or apparent from the following description and drawings.

SUMMARY OF THE INVENTION

The general area of this invention relates to porous materials made fromnanofiber packed beds. More particularly, the invention relates toaltering the porosity or packing structure of a nanofiber packed bedstructure by blending nanofibers with scaffold particulates havinglarger dimensions. For example, adding large diameter fibers to ananotube packed bed to serve as a scaffolding to hold the smallernanofibers apart and prevent the nanofiber bed structure fromcollapsing. This increases the average pore size of the mass by changingthe pore size distribution and alters the packing structure of thepacked bed. The increase in average pore size is caused by the creationof larger channels which improves the flow of liquids or gasses throughthese materials.

Accordingly, the purpose of the invention is to alter the average poresize and packing structure of packed layers of nanofibers by blending inlarger particulates, preferably fibers having larger diameters. Thelarger particulates alter the packing of the nanofibers and lead tostructures with reduced resistance to fluid flow. The present inventionprovides the unexpected advantage of being able to form a packed bedstructure from nanofibers with enhanced fluid flow characteristics as aresult of the scaffolding effect provided by the scaffold particulates.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a photomicrograph (magnification of ×50,000) of a nanofiberpacked bed illustrating a nanofiber mat region comprising randomlyoriented intertwined carbon fibrils.

FIG. 2 is a photomicrograph (magnification of ×2,000) illustrating apacked bed comprising scaffold fiber particulates and web-likestructures of randomly oriented intertwined carbon fibrils.

FIG. 3 is a photomicrograph (magnification of ×200) illustratingscaffold particulates in the form of fibers (4-8 μm in diameter) andweb-like regions of fibril mats.

FIG. 4 is a photomicrograph (×100) of a packed bed structure accordingto the present invention.

FIG. 5 illustrates a graphical representation of the flow rate/matstructure relationship for comparative nanofiber mats (withoutscaffolding) wherein the vertical axis represents flow rate and thehorizontal axis represents the total volume flow through the mat formats with different thickness and densities.

FIG. 6 illustrates a graphical representation of the flow rate/matdensity relationship of a comparative carbon nanofiber mats (withoutscaffolding) wherein the vertical axis represents flow rate normalizedfor mat thickness and the horizontal axis represents mat density.

FIG. 7 illustrates a graphical representation of the flow rate/nanofiberfraction relationship of one embodiment of the invention wherein thevertical axis represents flow rate and the horizontal axis representsfraction of nanofibers by weight in a packing bed containing carbonfibrils and carbon fibers.

FIG. 8 is a graphical representation of the current-voltage relationshipfor the electrode of example 8 wherein the vertical axis representscurrent and the horizontal axis represents the applied potential atseveral potential scan rates.

DETAILED DESCRIPTION OF THE INVENTION Definitions

The term "fluid flow rate characteristic" refers to the ability of afluid or gas to pass through a solid structure. For example, the rate atwhich a volume of a fluid or gas passes through a three-dimensionalstructure having a specific cross-sectional area and specific thicknessor height (i.e. milliliters per minute per square centimeter per milthickness) at a fixed pressure differential through the structure.

The term "isotropic" means that all measurements of a physical propertywithin a plane or volume of the packed bed, independent of the directionof the measurement, are of a constant value. It is understood thatmeasurements of such non-solid compositions must be taken on arepresentative sample of the packed bed so that the average value of thevoid spaces is taken into account.

The term "macroscopic" refers to structures having at least twodimensions greater than 1 mm.

The term "nanofiber" refers to elongated structures having a crosssection (angular fibers having edges) or diameter (rounded) less than 1micron. The structure may be either hollow or solid. This term isdefined further below.

The term "packed bed", "assemblage" or "mat" refers to a structurecomprising a configuration of a mass of intertwined individualnanofibers, scaffold fibers and/or scaffold particulate matter. The term"packed bed" will hereafter be construed as including and beinginterchangeable with the terms "mats", "assemblages" and related threedimensional structures. The term "packed bed" does not include loosemasses of particulate matter.

The term "packing structure" refers to the internal structure of apacked bed including the relative orientation of the fibers, thediversity of and overall average of fiber orientations, the proximity ofthe fibers to one another, the void space or pores created by theinterstice and spaces between the fibers and size, shape, number andorientation of the flow channels or paths formed by the connection ofthe void space or pores. The term "relative orientation" refers to theorientation of an individual fiber with respect to the others (i.e.,aligned versus non-aligned). The "diversity of" and "overall average" offiber orientations refers to the range of fiber orientations within thepacked bed (alignment and orientation with respect to the externalsurface of the bed).

The term "physical property" means an inherent, measurable property ofthe porous packed bed, e.g. resistivity, fluid flow characteristics,density, porosity, etc.

The term "relatively" means that ninety-five percent of the values ofthe physical property when measured along an axis of, or within a planeof or within a volume of the structure, as the case may be, will bewithin plus or minus fifty percent of a mean value.

The term "scaffold particulate" refers to a particulate materialsuitable for providing a scaffolding effect when blended withnanofibers. At least one dimension of the "scaffold particulate" issubstantially greater than at least one dimension of the nanofibers. The"scaffold particulates" can have various three-dimensional shapesincluding fibers, cubes, platelets, discs, etc. "Scaffold particulates"are discussed further below.

The term "substantially" means that ninety-five percent of the values ofthe physical property when measured along an axis of, or within a planeof or within a volume of the structure, as the case may be, will bewithin plus or minus ten percent of a mean value.

The terms "substantially isotropic" or "relatively isotropic" correspondto the ranges of variability in the values of a physical property setforth above.

Nanofibers

The term nanofibers refers to various fibers having very small diametersincluding fibrils, whiskers, nanotubes, buckytubes, etc. Such structuresprovide significant surface area when incorporated in a packed bedstructure because of their size. Moreover, such structure can be madewith high purity and uniformity. Preferably, the nanofiber used in thepresent invention has a diameter less than about 1 micron, preferablyless than about 0.5 micron, and even more preferably less than 0.1micron and most preferably less than 0.05 micron.

The fibrils, buckytubes, nanotubes and whiskers that are referred to inthis application are distinguishable from continuous carbon fiberscommercially available as reinforcement materials. In contrast tonanofibers, which have, desirably large, but unavoidably finite aspectratios, continuous carbon fibers have aspect ratios (L/D) of at least10⁴ and often 10⁶ or more. The diameter of continuous fibers is also farlarger than that of fibrils, being always >1.0 μm and typically 5 to 7μm.

Continuous carbon fibers are made by the pyrolysis of organic precursorfibers, usually rayon, polyacrylonitrile (PAN) and pitch. Thus, they mayinclude heteroatoms within their structure. The graphitic nature of "asmade" continuous carbon fibers varies, but they may be subjected to asubsequent graphitization step. Differences in degree of graphitization,orientation and crystallinity of graphite planes, if they are present,the potential presence of heteroatoms and even the absolute differencein substrate diameter make experience with continuous fibers poorpredictors of nanofiber chemistry.

The various types of nanofibers suitable for use in porous packed bedstructures are discussed below.

Carbon fibrils are vermicular carbon deposits having diameters less than1.0 μm, preferably less than 0.5 μm, even more preferably less than 0.2μm and most preferably less than 0.05 μm. They exist in a variety offorms and have been prepared through the catalytic decomposition ofvarious carbon-containing gases at metal surfaces. Such vermicularcarbon deposits have been observed almost since the advent of electronmicroscopy. A good early survey and reference is found in Baker andHarris, Chemistry and Physics of Carbon, Walker and Thrower ed., Vol.14, 1978, p. 83 and Rodriguez, N., J. Mater. Research, Vol. 8, p. 3233(1993), each of which are hereby incorporated by reference. (see also,Obelin, A. and Endo, M., J. of Crystal Growth, Vol. 32 (1976), pp.335-349, hereby incorporated by reference).

U.S. Pat. No. 4,663,230 to Tennent, hereby incorporated by reference,describes carbon fibrils that are free of a continuous thermal carbonovercoat and have multiple ordered graphitic outer layers that aresubstantially parallel to the fibril axis. As such they may becharacterized as having their c-axes, the axes which are perpendicularto the tangents of the curved layers of graphite, substantiallyperpendicular to their cylindrical axes. They generally have diametersno greater than 0.1 μm and length to diameter ratios of at least 5.Desirably they are substantially free of a continuous thermal carbonovercoat, i.e., pyrolytically deposited carbon resulting from thermalcracking of the gas feed used to prepare them. The Tennent inventionprovided access to smaller diameter fibrils, typically 35 to 700 Å(0.0035 to 0.070μm) and to an ordered, "as grown" graphitic surface.Fibrillar carbons of less perfect structure, but also without apyrolytic carbon outer layer have also been grown.

U.S. Pat. No. 5,171,560 to Tennent et al., hereby incorporated byreference, describes carbon fibrils free of thermal overcoat and havinggraphitic layers substantially parallel to the fibril axes such that theprojection of said layers on said fibril axes extends for a distance ofat least two fibril diameters. Typically, such fibrils are substantiallycylindrical, graphitic nanotubes of substantially constant diameter andcomprise cylindrical graphitic sheets whose c-axes are substantiallyperpendicular to their cylindrical axis. They are substantially free ofpyrolytically deposited carbon, have a diameter less than 0.1 μm and alength to diameter ratio of greater than 5. These fibrils are of primaryinterest in the invention.

Further details regarding the formation of carbon fibril aggregates maybe found in the disclosure of U.S. Pat. No. 5,165,909 to Tennent; Snyderet al., U.S. Pat. application Ser. No. 149,573, filed Jan. 28, 1988, andPCT application No. US89/00322, filed Jan. 28, 1989 ("Carbon Fibrils")WO 89/07163, and Moy et al., U.S. Patent application Ser. No. 413,837filed Sep. 28, 1989 and PCT application No. US90/05498, filed Sep. 27,1990 ("Fibril Aggregates and Method of Making Same") WO 91/05089, andU.S. application Ser. No. 08/479,864 to Mandeville et al., filed Jun. 7,1995 and U.S. application Ser. No. 08/329,774 by Bening et al., filedOctober 27, 1984 and U.S. application Ser. No. 08/284,917, filed Aug. 2,1994 and U.S. application Ser. No. 07/320,564, filed Oct. 11, 1994 byMoy et al., all of which are assigned to the same assignee as theinvention here and are hereby incorporated by reference.

Moy et al., U.S. application Ser. No. 07/887,307 filed May 22, 1992,hereby incorporated by reference, describes fibrils prepared asaggregates having various morphologies (as determined by scanningelectron microscopy) in which they are randomly entangled with eachother to form entangled balls of fibrils resembling bird nests ("BN");or as aggregates consisting of bundles of straight to slightly bent orkinked carbon fibrils having substantially the same relativeorientation, and having the appearance of combed yarn ("CY") e.g., thelongitudinal axis of each fibril (despite individual bends or kinks)extends in the same direction as that of the surrounding fibrils in thebundles; or, as, aggregates consisting of straight to slightly bent orkinked fibrils which are loosely entangled with each other to form an"open net" ("ON") structure. In open net structures the degree of fibrilentanglement is greater than observed in the combed yarn aggregates (inwhich the individual fibrils have substantially the same relativeorientation) but less than that of bird nests. CY and ON aggregates aremore readily dispersed than BN making them useful in compositefabrication where uniform properties throughout the structure aredesired.

When the projection of the graphitic layers on the fibril axis extendsfor a distance of less than two fibril diameters, the carbon planes ofthe graphitic nanofiber, in cross section, take on a herring boneappearance. These are termed fishbone fibrils. Geus, U.S. Pat. No.4,855,091, hereby incorporated by reference, provides a procedure forpreparation of fishbone fibrils substantially free of a pyrolyticovercoat. These fibrils are also useful in the practice of theinvention.

McCarthy et al., U.S. Pat. application Ser. No. 351,967 filed May 15,1989, hereby incorporated by reference, describes processes foroxidizing the surface of carbon fibrils that include contacting thefibrils with an oxidizing agent that includes sulfuric acid (H₂ SO₄) andpotassium chlorate (KClO₃) under reaction conditions (e.g., time,temperature, and pressure) sufficient to oxidize the surface of thefibril. The fibrils oxidized according to the processes of McCarthy, etal. are non-uniformly oxidized, that is, the carbon atoms aresubstituted with a mixture of carboxyl, aldehyde, ketone, phenolic andother carbonyl groups.

Fibrils have also been oxidized non-uniformly by treatment with nitricacid. International Application PCT/US94/10168 discloses the formationof oxidized fibrils containing a mixture of functional groups.

In published work, McCarthy and Bening (Polymer Preprints ACS Div. ofPolymer Chem. 30 (1)420(1990)) prepared derivatives of oxidized fibrilsin order to demonstrate that the surface comprised a variety of oxidizedgroups. The compounds they prepared, phenylhydrazones,haloaromaticesters, thallous salts, etc., were selected because of theiranalytical utility, being, for example, brightly colored, or exhibitingsome other strong and easily identified and differentiated signal. Thesecompounds were not isolated and are, unlike the derivatives describedherein, of no practical significance.

Carbon nanotubes of a morphology similar to the catalytically grownfibrils described above have been grown in a high temperature carbon arc(Iijima, Nature 354 56 1991, hereby incorporated by reference). It isnow generally accepted (Weaver, Science 265 1994, hereby incorporated byreference) that these arc-grown nanofibers have the same morphology asthe earlier catalytically grown fibrils of Tennent. Arc grown carbonnanofibers are also useful in the invention.

Scaffold Particulates

Scaffold particulates are particulate solids having a shape and sizesuitable to providing a scaffolding effect when blended with nanofibers.The scaffold particulates are of a shape and size such that they disruptthe packing structure of the nanofibers. This results in a packed bedhaving an increased average pore size. The scaffolding increases thenumber of large pores and the average pore size, which in turn increasesthe flow rate of the bed. The scaffold particulates are used as adiluent and/or as a mechanically stronger scaffolding that helpsovercome the forces of surface tension during the drying process whichreduces the density of the nanofiber fraction of the resulting composite"mat".

Preferably, the scaffold particulates have at least one dimension largerthan the largest dimension of the nanofibers, and/or at least a secondlargest dimension larger than the second largest dimension of thenanofiber. The largest dimension of the scaffold particle may becomparable to the largest dimension of the nanofiber. For example,nanofibers and fat fibers of the same length may be used as long as thediameters of the fat fibers are significantly larger than the diametersof the nanofibers.

Preferably, the largest dimension of the scaffold particulate is atleast 10 times larger than the largest dimension of the nanofibers, morepreferably 50 times greater, even more preferably 100 times greater, andmost preferably 200 times greater.

Preferably, the second largest dimension of the scaffold particulate isat least 10 times larger than the second largest dimension of thenanofibers, more preferably 50 times greater, even more preferably 100times greater, and most preferably 200 times greater.

The scaffold particulates also preferably have a largest dimension(e.g., length for a fiber) greater than 1 micron, more preferablygreater than 5 microns, even more preferably greater than 10 microns andmost preferably greater than 50 microns. The scaffold particulatespreferably have a second largest dimension (e.g. diameter for a fiber orthickness for a disc) greater than 0.1 micron, more preferably greaterthan 1 micron, even more preferably greater than 3 microns and mostpreferably greater than 5 microns.

The shape of the scaffold particulates can take many forms as long asthe particulate provides a sufficient scaffolding effect to enhance thefluid flow characteristics by the desired amount. Suitable shapesinclude fibers, platelets, discs, cones, pyramids, cubes, irregularsolid particulates, etc.

Preferably, the scaffold particulate has a shape of a fiber. Thediameter of the scaffold particulate fiber is preferably at least 10times greater than the nanofiber diameter, more preferably 50 timesgreater, even more preferably 100 times greater and most preferably 200times greater.

The surface chemistry, structure or composition of the scaffoldparticulate may also be modified to enhance or adjust the scaffoldingeffect. For example, particulates having rough surfaces may result inincreased scaffolding since such particulates would be better able tohold the nanofibers apart. According to one embodiment of the invention,more that one type of scaffold particulate is incorporated into thepacked bed.

The scaffold particulates may be, for example, polymeric, inorganic,glass or metallic. The particulate can have the same or differentcomposition than the nanofiber. Preferably, the scaffold particulatesare either glass fiber particles or carbon fibers.

According to a preferred embodiment, carbon fibers are used as thescaffold particulate. The carbon fibers offer the advantage for thispurpose of being conductive. Metal fibers, with even greaterconductivity, can also be blended in for the same purpose. Accordingly,carbon fibers, which are conductive, but also made of carbon, may beused as the scaffold particulates and mixed with carbon nanofibers toresult in a carbon based packed bed having improved conductivity,enhanced fluid flow and high carbon purity.

Nanofiber Packed Beds having Enhanced Fluid Flow

The general area of this invention relates to porous packed bedmaterials made from packed nanofibers. The invention relates to a methodfor altering the porosity of a packed bed structure made by blendingnanofibers with other, larger diameter fibers or particulate materialsto serve as a scaffolding to hold the smaller nanofibers apart andprevent them from collapsing. The creation of larger channels withinthese composite materials improves the flow of liquids or gasses throughthese materials.

The flow of a fluid through a capillary is described by Poiseuille'sequation which relates the flow rate to the pressure differential, thefluid viscosity, the path length and size of the capillaries. The rateof flow per unit area varies with the square of the pore size.Accordingly, a pore twice as large results in flow rates four times aslarge. The creation of pores of a substantially larger size in thenanofiber packed bed structure results in increased fluid flow becausethe flow is substantially greater through the larger pores.

As set forth above, nanofiber packed beds not provided with ascaffolding mechanism results in poor fluid flow characteristics. Thisis particularly true when liquid suspensions of nanofibers are drainedof the suspending fluid and the surface tension of the liquid tends topull the nanofibers into a dense packed "mat" with the pore sizedetermined by the interfiber spaces which tend to be quite small. Thisresults in a packed bed having poor fluid flow characteristics.

When the nanofiber packed beds are formed without the scaffoldparticulates, they collapse and form dense mats. The scaffoldparticulates hold up the mats and break or tear apart these mats formingdiscontinuous nanofiber regions and void channels in between theseregions. Referring to FIG. 1, the microphotograph illustrates the densepacking of the nanofibers within the nanofiber mat region or domain.FIG. 2 is a microphotograph of lower magnification illustrating thenanofiber domains clinging to and forming "web-like" structures betweenand upon the scaffold particulates. The intertwined fibrous structuresof nanofibers cause them to cling together to form the web-like orfelt-like structure rather than break apart completely into discreteparticles. If non-fibrous nano particles (for example, spheres) wereused, these particles would simply fall through the structure andsegregate to the bottom. FIGS. 3 and 4 are lower magnificationmicrophotographs of packings containing carbon fibrils and carbonfibers.

Surprisingly, packed beds comprising larger scaffolding particulatesblended together with nanofibers offer improved flow characteristics.This permits the high surface area of the nanofibers to be more readilyutilized in situations where bulk fluid or gas transport through thematerial is needed.

The enhanced packing structure of the packed bed provides large flowchannels that enable a large surface area of nanofibers to beaccessible. That is, the nanofibers that line the outer walls of or arein contact with the large flow channels formed within the compositestructure allow an increased amount of accessible nanofiber surfacearea.

One aspect of the invention relates to a composition of mattercomprising a porous packed bed having a plurality of nanofibers and anumber of scaffold particulates blended to form a porous structure.Preferably, the packed bed has an enhanced fluid flow characteristic.Preferably, the fluid flow rate of water is greater than 0.5 ml/min/cm²at a pressure differential across the packed bed of about 1 atm for abed having a thickness of one mil, more preferably greater than 1.0 mlper min per cm².

Broadly, the invention is in a composition of matter consistingessentially of a three-dimensional, macroscopic assemblage of amultiplicity of randomly oriented nanofibers, blended with scaffoldparticulates. Preferably, the beds have at least two dimensions greaterthan 1 mm, more preferably greater than 5 mm. Preferably, the resultingpacked beds have a bulk density of from 0.1 to 0.5 gm/cc.

The packed bed made according to the invention have structural andmechanical integrity. That is, the beds can be handled without breakingor falling apart, although the beds may be somewhat flexible. The packedbeds have a felt-like character as a result of the nanofibersintertwining together to form a random woven-like structure. The packedbeds have significantly higher strengths and toughness compared tosimple loose particulate masses.

Preferably, the packed bed of the invention has at least one fluid flowcharacteristic that is greater than that of a nanofiber packed bedwithout said scaffold particulates. That is, the addition of thescaffold particulates enhances the fluid flow rate, for example,compared to a nanofiber packed bed without the scaffold particulates.Preferably, the scaffolding addition increases the flow rate by a factorof at least two, more preferably by a factor of at least five, even morepreferably by a factor of at least ten, and most preferably by a factorof at least 50.

According to one preferred embodiment, the packed bed has a surface areagreater than about 25 m² /g.

Preferably, the packed bed has relatively or substantially uniformphysical properties along at least one dimensional axis and desirablyhas relatively or substantially uniform physical properties in one ormore planes within the packed bed, i.e. they have isotropic physicalproperties in that plane. In other embodiments, the entire packed bed isrelatively or substantially isotropic with respect to one or more of itsphysical properties.

Preferably, the packed bed has substantially isotropic physicalproperties in at least two dimensions. When fibers or platelets are usedas the scaffold particulates, they tend to align in single plane.However, the resulting structure is isotropic in the plane of alignment.The packed bed may have a uniform or nonuniform distribution of scaffoldparticulates and nanofibers.

According to one embodiment of the invention, the distribution ofscaffold particulates within the nanofiber packed bed is nonuniform. Ascan be seen from comparing FIGS. 1, 2, 3 and 4, although thedistribution may appear uniform at lower magnifications, highmagnifications indicate the nanofibers may congregate and form web-likedomains. Although this may occur, the macroscopic properties of thematerial can be relatively uniform as shown in FIG. 4. Alternatively,the packed structure may be non-uniform. For example, a thin layerregion having a higher concentration of nanotubes may be formed on thetop portion of the packed bed. Alternatively, the thin layer ofnanofibers may be formed at a bottom portion or within the packed bedstructure. The distribution of the scaffold particulates may be variedto alter the properties of the packed bed.

The average pore size and overall packing structure of the packed bedcan be adjusted by varying several parameters. These parameters include:(a) the weight percent of nanofiber and/or support particulate, (b) thesize, shape and surface characteristics of the nanofiber and/or scaffoldparticulate, (c) the composition of the nanofiber and/or scaffoldparticulate (e.g., carbon vs. metal), and (d) the method of making thepacked bed.

Although the interstices between the nanofibers are irregular in bothsize and shape, they can be thought of as pores and characterized by themethods used to characterize porous media. The size distribution of theinterstices and void space within in such networks can be controlled bythe concentration and level of dispersion of nanofibers blended with thescaffold supports. The addition of the scaffolds to the nanofiberpackings causes an altering of the pore size distribution. Although thepore size within the nanofiber mat domains are not significantlyaltered, the breaking apart of these domains results in larger voidchannels between the nanofiber domains. The result is a bimodal poresize distribution. Small pure spaces with the nanofiber domains andlarge void space between these domains.

According to one embodiment, the porous packed beds may contain anamount of nanofibers ranging from 1 weight percent to 99 weight percent,preferably 1 weight percent to 95 weight percent, even more preferably 1weight percent to 50 weight percent, and most preferably 5 weightpercent to 50 weight percent. The corresponding amount of scaffoldparticulate ranges from 99 to 1 weight percent, more preferably 99 to 5weight percent, even more preferably 99 to 50 weight percent and mostpreferably 95 to 50 weight percent.

According to another embodiment, the porous packed beds may comprise anon-void solid volume having an amount of nanofibers ranging from 1volume percent to 99 volume percent, preferably 1 volume percent to 95volume percent, even more preferably 1 volume percent to 50 volumepercent, and most preferably 5 volume percent to 50 volume percent. Thecorresponding amount of scaffold particulate ranges from 99 to 1 volumepercent (of non-void solid volume), more preferably 99 to 5 volumepercent, even more preferably 99 to 50 volume percent and mostpreferably 95 to 50 volume percent.

As discussed above, varying the amount of nanofiber and scaffoldparticulate varies the packing structure, and accordingly varies thefluid flow characteristics. FIG. 5 is a graphical representation of therelationship between water fluid flow and mat thickness for a carbonfibril mat (2 cm²) without scaffold particulates. FIG. 6 illustrates agraphical representation of the flow rate vs. mat density. As can beseen from the graph, the fluid flow characteristics for these mats arepoor. The flow rate decreases exponentially as the mat density isincreased, corresponding to a decrease in pore volume. FIG. 7 is agraphical representation of the fluid flow rate/nanofiber fractionrelationship for a packed bed (2 cm²) made according to the presentinvention. As can be seen from the graph, increased flow rates areachieved by adding scaffold fibers to a nanofiber mat.

According to one preferred embodiment of the invention, the nanofibershave an average diameter less than 0.1 micron and said scaffoldparticulates have a first dimension greater than about 1 micron and asecond dimension greater than about 0.5 micron.

According to one embodiment of the invention, the packed bed furthercomprises an additive (e.g. particle additive) as a third componentincorporated within the bed in an amount ranging from 0.01 wt % to 50 wt%, more preferably 0.01 to 20 wt %, even more preferably 0.01 to 10 wt %and most preferably from 0.01 to 5 wt %.

Methods of Making Nanofiber Packed Beds

Generally, the method involves blending the nanofibers with the scaffoldparticulates to form a packed bed structure. Suitable comparable methodsare set forth in U.S. application Ser. No. 08/428,496 filed Apr. 27,1995, hereby incorporated by reference.

According to one embodiment of the invention, a porous composite packedbed having a plurality of nanofibers and a number of scaffoldparticulates is prepared by a method comprising the steps of:

(a) dispersing the nanofibers and the scaffold particulates in a mediumto form a suspension; and

(b) separating the medium from the suspension to form the packed bed.

According to one preferred embodiment of the invention, packed beds wereformed using either glass fibers from commercial glass fibers (e.g.,Whatman® GF/B glass fiber filter paper) or chopped carbon fibers as thescaffold particulates. In both cases, nanofibers (graphite fibrils) andthe larger fibers were mixed together to form suspensions of the two andfiltered on a vacuum filter manifold to remove the fluid and form a dry,packed "mat" or bed.

The medium is preferably selected from the group consisting of water andorganic solvents.

The step of separating may comprise filtering the suspension orevaporating the medium from said suspension.

According to one embodiment, the suspension is a gel or paste comprisingthe nanofibers and scaffold particulates in a fluid. The step ofseparating may comprise the steps of:

(a) heating the gel or paste in a pressure vessel to a temperature abovethe critical temperature of said fluid;

(b) removing supercritical fluid from the pressure vessel; and

(c) removing said porous composite packed bed from the pressure vessel.

According to another embodiment, a slurry of carbon nanotubes isprepared and chopped or milled segments of support particulates areadded to a suspension containing the nanofibers and agitated to keep thematerials dispersed. The medium is subsequently separated from thesuspension forming a packed bed having good fluid flow characteristics.Support fiber particulates of 1/8 inch in length and even smaller workwell.

Methods of Using Nanotube Packed Beds

The fibril packed beds may be used for purposes for which porous mediaare known to be useful. These include filtration, electrodes,adsorbants, chromatography media, etc. In addition, the packed beds area convenient bulk form of nanofibers and may thus be used for any knownapplications including especially EMI shielding, polymer composites,active electrodes, etc.

For some applications like EMI shielding, filtration and currentcollection, unmodified nanofiber packed beds can be used. For otherapplications, the nanofiber packed beds are a component of a morecomplex material, i.e. they are part of a composite. Examples of suchcomposites are polymer molding compounds, chromatography media,electrodes for fuel cells and batteries and ceramic composites,including bioceramics like artificial bone. In some of these composites,like molding compounds and artificial bone, it is desirable that thenon-nanofiber components fill--or substantially fill--the porosity ofthe packed bed. For others, like electrodes and chromatography media,their usefulness depends on the composite retaining at least some of theporosity of the packed bed.

The rigid networks can also serve as the backbone in biomimetic systemsfor molecular recognition. Such systems have been described in U.S. Pat.No. 5,110,833 and International Patent Publication No. WO93/19844.

The above described packed bed products may be incorporated in a matrix.Accordingly, a non-nanofiber component is filtrated through the bed andsolidified to form a composite. Preferably, the matrix is an organicpolymer (e.g., a thermoset resin such as epoxy, bismaleimide, polyamide,or polyester resin; a thermoplastic resin; a reaction injection moldedresin; or an elastomer such as natural rubber, styrene-butadiene rubber,or cis-1,4-polybutadiene); an inorganic polymer (e.g., a polymericinorganic oxide such as glass), a metal (e.g., lead or copper), or aceramic material (e.g., Portland cement).

Packed Beds as Electrodes

According to one preferred embodiment, the above-described packed bedsare used as and have been shown to be good porous electrode materials.Accordingly, another aspect of the invention relates to flow throughelectrode materials comprising the nanofiber packed beds having enhancedfluid flow characteristics. The porous electrode material is made fromthe packed bed mixtures of nanofibers and scaffold particulates. Theelectrode materials can take advantage of the composite properties ofdifferent materials in the composite mix. Moreover, the conductivity ofthe assemblage can be increased when mixing various conductive scaffoldparticulates with the nanofibers.

According to one embodiment, electrodes may be made by attaching aconductive wire to sections of the packed beds with conductive epoxy.Such electrodes may be examined by cyclic voltammetry. It was observedthat the material filtered better than the fibrils would have alone.

According to another embodiment, an electrical contact is formed byforming the packed bed on a conductive surface. For example, the packedbed may be formed by filtering the packed bed onto a conductive mesh.Preferably, a gold, platinum, nickel or stainless steel mesh or screenis used.

Moreover, the conductivity may be limited by the inner connection of thenanofibers. Composites having larger fibers blended in with thenanofibers offers improved porosity and/or flow characteristics as wellas conductivity. This permits the surface area of the nanofibers to bemore readily utilized in electrode applications in situations where bothconductivity and fluid flow through the material are needed.

The addition of different amounts of larger scaffold particulates with afixed amount of nanofibers allows the internal volume to be fine"tuned". The small pore sizes within the structure results in thin filmelectrochemical behavior, the effective "thickness" of the film layereffect can be varied. Thus porous, highly conductive electrodes can bemade with different effective void volumes.

EXAMPLES

The following examples are illustrative of some of the products andmethods of making the same falling within the scope of the presentinvention. They are, of course, not to be considered in any waylimitative of the invention. Numerous changes and modification can bemade with respect to the invention.

EXAMPLE I (Comparative) Preparation of Fibril Mats Using Prior Methods

A dilute dispersion of fibrils is used to prepare porous mats or sheets.A suspension of fibrils is prepared containing 0.5% fibrils in waterusing a Waring blender. After subsequent dilution to 0.1%, the fibrilsare further dispersed with a probe type sonifier. The dispersion is thenvacuum filtered to form a mat, which is then oven dried.

The mat has a thickness of about 0.20 mm and a density of about 0.20gm/cc corresponding to a pore volume of 0.90. The electrical resistivityin the plane of the mat is about 0.02 ohm/cm. The resistivity in thedirection perpendicular to the mat is about 1.0 ohm/cm. The fluid flowcharacteristics of the mat are poor.

EXAMPLE II (Comparative) Preparation of Fibril Mats Using Prior Methods

A suspension of fibrils is prepared containing 0.5% fibrils in ethanolusing a Waring Blender. After subsequent dilution to 0.1%, the fibrilsare further dispersed with a probe type sonifier. The ethanol is thenallowed to evaporate and a mat is formed. The fluid flow characteristicsof the mat are poor.

EXAMPLE III (Comparative) Preparation of Porous Fibril Plugs Using PriorMethods

Supercritical fluid removal from a well dispersed-fibril paste is usedto prepare low density shapes. 50 cc of a 0.5% dispersion in n-pentaneis charged to a pressure vessel of slightly larger capacity which isequipped with a needle valve to enable slow release of pressure. Afterthe vessel is heated above the critical temperature of pentane (Tc=196.6C. °), the needle valve is cracked open slightly to bleed thesupercritical pentane over a period of about an hour.

The resultant solid plug of Fibrils, which has the shape of the vesselinterior, has a density of 0.005 g/cc, corresponding to a pore volumefraction of 0.997%. The resistivity is isotropic and about 20 ohm/cm.The mat has the poor fluid flow characteristics.

EXAMPLE IV (Inventive) Preparation of composite mats of Fibrils andGlass fibers

A suspension of fibrils was prepared by mixing 2 grams of fibrils in 400mils of DI water (0.5%, w/w) and blending in a Waring® blender for 5minutes on high. 60 mls of the suspension was diluted to 200 mls with DIwater and sonicated with a 450 Watt Branson® probe sonicator for 13minutes on high power with a 100% duty cycle to make a stock fibrildispersion. Disks (0.25 inch diameter, 6.6 mg/disk) were punched from aWhatman® GF/B fiberglass filter. Aliquots of the fibril stock dispersionand fiberglass disks were mixed in the proportions indicated in Table 1:

                  TABLE 1                                                         ______________________________________                                        Suspensions Containing Carbon Nanotubes and Fiberglass Particulates                   Carbon Nanotube                                                                              Fiberglass                                             Composite #                                                                           Suspension     (No. of Disks)                                                                           DI water                                    ______________________________________                                        184-20-1                                                                              20 mls          0         80 mls                                      184-20-2                                                                              20 mls         10         80 mls                                      184-20-3                                                                              20 mls         20         80 mls                                      184-20-4                                                                              20 mls         30         80 mls                                      184-20-5                                                                              20 mls         40         80 mls                                      ______________________________________                                    

Each 100 ml mixture was sonicated for an additional 4 minutes on highpower and filtered onto a 0.45 μm MSI nylon filter in a 47 mm Milliporemembrane filter manifold. The fibril/fiberglass composites formedfelt-like mats which were peeled off of the nylon membranes and driedfor several hours at 80° C. between two pieces of filter paper underweight to maintain flatness. The area of each mat disk, defined by thedimensions of the filtration manifold, was 10 cm². Each mat was weighedand the thickness measured with calipers. The data as listed in Table 2:

                                      TABLE 2                                     __________________________________________________________________________    Mixed Fiberglass/CC Fibril Mat Data                                                Weight                                                                            Area                                                                             Height                                                                            Glass Fibril                                                                             Glass                                                                             Fibril                                                                            Overall                                    Mat No.                                                                            (mgs)                                                                             (cm.sup.2)                                                                       (mils)                                                                            Wt. (mgs)*                                                                          Wt. (mgs)                                                                          d" g/cc                                                                           d" g/cc                                                                           g/cc                                       __________________________________________________________________________    184-20-1                                                                           35.3                                                                              10 5.5 0.0   35.3 0.00                                                                              0.25                                                                              0.25                                       184-20-2                                                                           101.37                                                                            10 11.5                                                                              66.0  35.4 0.23                                                                              0.12                                                                              0.35                                       184-20-3                                                                           170.75                                                                            10 21.5                                                                              132.0 38.8 0.24                                                                              0.07                                                                              0.31                                       184-20-4                                                                           292.85                                                                            10 35.0                                                                              256.9  36.0**                                                                            0.29                                                                              0.04                                                                              0.33                                       184-20-5                                                                           554.38                                                                            10 120.0                                                                             518.4  36.0**                                                                            0.17                                                                              0.01                                                                              0.18                                       __________________________________________________________________________     *6.6 mg/disk                                                                  **set to 36 mg total cc.                                                      Glass d" and Fibril d" refer to the total weight of fibril or fiber glass     divided by the total volume of the mat.                                  

The flow characteristics of each mat were measured by monitoring waterflow through each mat in a 25 mm diameter membrane filter manifoldconnected to a vacuum pump capable of pulling a vacuum close to 29" ofHg. The 10 cm² mats were centered and clamped into the 25 mm diameterfilter manifold insuring that fluid could only flow through the mat andnot around any outside edge. The DI water used for the flow studies wasfiltered sequentially through a 0.45 μm pore size nylon filter, a 0.2 μmpore size cellulose membrane filter and finally a 0.1 μm pore sizecellulose membrane filter to remove traces of materials that mightinterfere with the flow studies. For each mat, the reservoir on themanifold was filled above the 15 milliliter volume line and vacuumapplied to establish flow. When the meniscus at the top of water levelcrossed the 15 ml mark a timer was started and the time recorded as thefluid level receded to lower level marks until the 5 ml level mark wasreached. The active filtration area was approximately 2 cm². In this waythe fluid flow was measured at several time points during the passage of10 mls of water. This procedure was followed for each composite mat aswell as for a sheet of Whatman® GF/B fiberglass filter. The flow dataare listed in Table 3:

                  TABLE 3                                                         ______________________________________                                        Measured Times* vs. Volume Flowed for Fibril/Fiberglass Mats                  Level                                                                         mls  GF/B   184-20-1 184-20-2                                                                             184-20-3                                                                             184-20-4                                                                             184-20-5                            ______________________________________                                        15   0.00   0.00     0.00   0.00   0.00   0.00                                14   --     2.23     3.97   1.02   0.13   --                                  13   --     5.02     8.07   2.47   0.29   0.08                                12   --     7.53     --     3.57   0.45   --                                  11   --     10.02    --     4.77   0.58   0.17                                10   --     12.45    --     5.85   0.72   --                                   9   --     --       --     6.87   0.87   0.25                                 8   --     --       --     7.98   1.00   --                                   7   --     --       --     8.93   --     0.34                                 6   --     22.45    --     10.00  1.29   --                                   5   0.03   24.98    34.30  11.02  1.44   0.43                                ______________________________________                                         "--" indicates not measured                                                   *times in minutes                                                        

For each filter mat the flow vs time was linear indicating that the matswere not being clogged and that the flow rate was constant over time.Also observed is that the flow rate increases with increasingproportions of glass fibers in the composite mats. As seen from the datain Table 3, the flow rate observed for composite mat no. 184-20-5 isnearly two orders of magnitude higher than for mat no. 184-20-1, a matcomposed of fibrils alone.

EXAMPLE V (Inventive) Preparation of composite mats of Fibrils andCarbon fibers

Composite mats of fibrils and chopped carbon fibers were prepared andexamined for flow properties. A suspension of fibrils was prepared bymixing two grams of fibrils in 400 mls of DI water (0.5%, w/w) andblending in a Waring® blender for 5 minutes on high. 70 mls of thesuspension was added to 280 mls of DI water, a magnetic stir bar wasadded and the suspension was subjected to 20 minutes of sonication with450 Watt Bransonic® probe sonicator with constant stirring to morecompletely disperse the fibrils and prepare 350 mls of 0.1% w/w stockdispersion. Weighed amounts of chopped carbon fibers (1/8 inch lengthchopped, Renoves, ex-PAN fibers) were placed in a series of beakers(seven) in the proportions shown in Table 4.

                  TABLE 4                                                         ______________________________________                                        Sample No.                                                                             WT 1/8" Chopped CF                                                                           H.sub.2 O                                                                              Fibril Solution                              ______________________________________                                        6         0 mg          100 mls  50 mls                                       7         50 mg         100 mls  50 mls                                       8         75 mg         100 mls  50 mls                                       9        100 mg         100 mls  50 mls                                       10       125 mg         100 mls  50 mls                                       11       150 mg         100 mls  50 mls                                       12       200 mg         100 mls  50 mls                                       ______________________________________                                    

100 mls of DI water and 50 mls of the 0.1% fibril stock dispersion wasadded to each beaker. Each of the seven solutions was sonicated for 5minutes with stirring and filtered onto a 0.45 μm nylon filter in a 47mm Millipore membrane filter manifold. The mats were left on themembranes, placed between pieces of filter paper and dried overnight at80° C. between two pieces of filter paper under a weight to maintainflatness. The weight and thickness of each 10 cm² mat was measured andthe data (after subtracting for the nylon membrane weight of 93.1 mgsand thickness of 4.0 mils) are shown in Table 5 along with a listing ofthe fractional composition of each of the mats.

                  TABLE 5                                                         ______________________________________                                        6        7       8       9     10    11    12                                 ______________________________________                                        Wt.   52.6   100.4   131.8 157.3 180.6 204.3 248.1                            mgs                                                                           Mils  10.0   20.5    27.0  36.0  41.0  54.0  66.0                             Wt. GF                                                                              52.6   53.0    53.0  53.0  53.0  53.0  53.0                             Wt. CF                                                                              0.0    48.4    78.8  104.3 127.6 151.3 195.1                            % GF  100%   52%     40%   34%   29%   26%   21%                              % CF   0%    48%     60%   66%   71%   74%   79%                              d' GF 0.21   0.10    0.08  0.06  0.05  0.40  0.03                             d' CF 0.00   0.09    0.12  0.12  0.12  0.11  0.12                             d Total                                                                             0.21   0.20    0.20  0.17  0.18  0.15  0.15                             ______________________________________                                         GF = graphic fibrils                                                          d' GF = weight of fibrils divided by total volume of mat                      d' CF = weight of carbon fibers divided by total volume of mat           

The fibril/carbon fiber mats, still on the nylon membranes, were usedfor flow studies as described in the previous example and the flow dataare shown in Table 6. The flow through of the 0.45 μm nylon filter byitself is very high. Accordingly, its presence does not significantlyalter the flow measurements of the supported mats.

                  TABLE 6                                                         ______________________________________                                        Level (mls)                                                                           6       7       8     9     10   11   12                              ______________________________________                                        15      0       0       0     0     0    0    0                               14      2.0     8.5     8.6   2.0   0.8  0.4  0.2                             13      4.1     14.3    16.6  3.9   1.8  0.7  0.5                             12      6.1     19.0    24.6  5.6   2.8  1.0  0.7                             11      --      22.8    32.1  7.2   3.7  1.3  0.9                             10      --      26.8    39.2  8.8   4.5  1.6  1.2                              9      --      30.8    46.2  10.5  5.4  1.9  1.4                              8      --      35.3    54.0  12.1  6.3  2.3  1.6                              7      16.3    39.3    --    13.7  7.2  2.5  1.8                              6      --      44.2    --    15.2  8.1  2.9  2.1                              5      20.6    48.5    --    16.8  9.0  3.2  2.3                             ______________________________________                                    

For each filter mat the flow rate vs time was linear indicating that thefilters were not being clogged and that the flow rate was constant overtime. It is observed that compared with a plain fibril mat, the flowrate shows a slight decrease with the two lower carbon fiberconcentrations but rises dramatically at higher carbon fiber levels.

EXAMPLE VI (Inventive) Use of Fibril/Fiberglass Composites Mats asElectrodes

Pieces of the fibril/fiberglass composite mats used in Example IV ofapproximately 5 mm by 8 mm were cut out of the mats with a razor bladeand fashioned into electrodes. The fashioning comprised connecting alength copper wire to one end of each 5 mm by 8 mm section with graphitepaint (Ladd Industries) and insulating the point of contact with epoxy.The copper wire was insulated in a glass sleeve and sealed with epoxy tothe mounted mat section such that only a 4 mm by 4 mm "flag" ofcomposite mat remained exposed. Two electrodes were prepared from eachmat as indicated in the table.

                  TABLE 7                                                         ______________________________________                                        Composite #   Electrode #                                                     ______________________________________                                        184-20-1      184-21-1, 2                                                     184-20-2      184-21-3, 4                                                     184-20-3      184-21-5, 6                                                     184-20-4      184-21-7, 8                                                     184-20-5       184-21-9, 10                                                   ______________________________________                                    

The composite electrodes were examined by cyclic voltammetry using anEG&G PAR 273 potentiostat, a Ag/AgCl reference electrode (BioanalyticalSystems, Inc.) and a Pt gauze counter electrode in a single compartmentcell (Bioanalytical Systems, Inc.) filled with a solution containing 3mM potassium ferricyanide, 3 mM potassium ferrocyanide and 0.5M K₂ SO₄in water. The ferri/ferrocyanide cyclic voltammograms exhibitedoxidation and reduction waves with characteristics that varied with thecomposition of the electrodes. Characteristic features of the cyclicvoltammograms recorded at a scan rate of 25 mv/second are listed inTable 8.

                  TABLE 8                                                         ______________________________________                                                                       E.sub.PA, V vs.                                                                      E.sub.PC, V vs.                         Electrode No.                                                                          Thickness*/Area**                                                                          I.sub.PA, mA                                                                           Ag/AgCl                                                                              Ag/AgCl                                 ______________________________________                                        184-21-1  5.5/0.140   0.6      0.275  0.225                                   184-21-3 11.5/0.215   1.2      0.295  0.215                                   184-21-5 21.5/0.187   1.4      0.310  0.21                                    184-21-7   35/0.165   2.7      0.380  0.14                                    184-21-9  120/0.210   2.7      0.550  -0.02                                   ______________________________________                                         *Thickness in Mils                                                            **Area in cm.sup.2                                                       

Electrode 184-21-1, consisting solely of fibrils showed very sharp peakswith minimal peak to peak separation consistent with a redox processtaking place within a porous electrode with very small pores. The peakshape and separation between anodic and cathodic peaks was similar towhat would be observed with a thin layer cell. The scan rate dependenceof the anodic peak current shows a nearly linear dependence forelectrode 184-21-1 but becomes increasingly nonlinear as the proportionof glass fibers in the electrode material increases. The anodic peakcurrents, in milliamps, recorded at different scan rates are shown inTable 9.

                                      TABLE 9                                     __________________________________________________________________________    Peak Anodic Current (I.sub.PA), mA vs. Scan Rate, mv/sec                      Scan Rate,                                                                         Pt wire                                                                  mv/sec                                                                             (1 cmx 0.05 cm)                                                                       184-21-1                                                                           184-21-3                                                                           184-21-5                                                                           184-21-7                                                                           184-21-9                                     __________________________________________________________________________     5   --      --   --   --   --   0.975                                        10   --      --   --   --   1.4  1.58                                         25   --      0.625                                                                              1.21 1.45 2.8  2.75                                         50   0.115   1.08 2.13 2.45 4.4  4.15                                         100  --      2.03 3.75 4.15 7    5.9                                          __________________________________________________________________________

EXAMPLE VII (Inventive) Use of Fibril/Carbon Fiber Composite Mats asElectrodes

Composite mats of fibrils and carbon fibers were used as electrodes.Carbon fibers are electrically conductive. Three fibril/carbon fibercomposite mats were prepared using the method described in the earlierexamples above. The proportions are as shown in Table 10.

                  TABLE 10                                                        ______________________________________                                        Composition of Fibril/Carbon Fiber Mats Used For Electrodes                                         Carbon Fiber                                                                            Carbon Nanotubes                              Composite Mat No.                                                                        Electrode Nos.                                                                           (mgs)     (mls of suspension)                           ______________________________________                                        184-22-1   184-28-1, 2                                                                              30        20                                            184-22-2   184-28-3   60        20                                            184-22-3   184-28-4, 5                                                                              120       20                                            ______________________________________                                    

Electrodes were prepared from fibril/carbon fiber mats using the methoddescribed in Example VI. The electrode dimensions are listed in Table 11below.

                  TABLE 11                                                        ______________________________________                                        Electrode     Area (cm.sup.2)                                                                        Thickness (mils)                                       ______________________________________                                        184-28-1      0.146    11                                                     184-28-2      0.118    11                                                     184-28-3      0.182    18                                                     184-28-4      0.175    37                                                     184-28-5      0.193    37                                                     ______________________________________                                    

The composite electrodes were examined by cyclic voltammetry using anEG&G PAR 273 potentiostat, a Ag/AgCl reference electrode (BioanalyticalSystems, Inc.) and a Pt gauze counter electrode in a single compartmentcell (Bioanalytical Systems, Inc.) filled with approx. 15 mls. of asolution containing 3 mM potassium ferricyanide, 3 mM potassiumferrocyanide and 0.5M K₂ SO₄ in water. The ferri/ferrocyanide cyclicvoltammograms exhibited an oxidation and reduction waves withcharacteristics that varied with the composition of the electrodes.Characteristic features of the cyclic voltammograms are listed in Table12.

                  TABLE 12                                                        ______________________________________                                        Summary of Cyclic Voltammetry Data of Ferri/Ferrocyanide at                   Fibril/Carbon Fiber Mat Electrodes                                            Parameter    Pt wire  184-28-1 184-28-3                                                                             184-28-4                                ______________________________________                                        E.sub.PA     0.32     0.23     0.290  0.34                                    E.sub.PC     0.15     0.29     0.215  0.17                                    E.sub.P-P    170      60       75     170                                     I.sub.PA at 5 mv/sec, mA                                                                   --       --       0.69   --                                      I.sub.PA at 10 mv/sec, mA                                                                  --       0.375    1.2    2.9                                     I.sub.PA at 25 mv/sec, mA                                                                  0.0663   0.80     2.55   5.3                                     I.sub.PA at 50 mv/sec, mA                                                                  0.0875   1.38     4.3    8.2                                     I.sub.PA at 100 mv/sec, mA                                                                 0.108    2.38     7.25   12.3                                    ______________________________________                                         E.sub.PA = anodic peak potential at 25 mv/sec, V vs Ag/AgCl                   E.sub.PC = cathodic peak potential at 25 mv/sec, V vs Ag/AgCl                 E.sub.P-P = peak to peak potential separation at 25 mv/sec, mv                I.sub.PA = anodic peak current                                           

All three of the carbon nanotube/carbon fiber composite electrodesshowed very sharp oxidation and reduction peaks with minimal peak topeak separation consistent with a redox process taking place withinporous electrodes having very small pores. The peak shape and separationbetween anodic and cathodic peaks is similar to that observed for a thinlayer cell. The scan rate dependence of the anodic peak current shows anearly linear dependence for electrode 184-28-1, but some deviation fromlinear dependence is observed for electrodes 184-28-3 and 184-28-4. Eachcomposite mat has the same amount of carbon nanotubes per unit area, butthe addition of the carbon fibers to the composite results in anincrease in thickness and hence electrode volume. The peak currents andintegrated currents increase with the electrode volume as expected duethe increased amounts of ferri/ferrocyanide solution within the porouselectrode.

These results demonstrate that the porosity of the fibril mat electrodescan be modified through the formation of composites with larger diameterfibers and enabling access to greater amounts of material in thesolution phase. Further, the use of conducting carbon fibers retains theconductive nature of the fibril mats as the fibrils in the mats arediluted with the larger carbon fibers in the composite.

The carbon fibers have a diameter of approx. 7-8 μm and the fibrils havea diameter of approx. 0.01 μm. Thus even though the fibril mat used tomake electrode 184-28-4 contains 80% carbon fibers (w/w) the carbonfibers contribute little to the total electrode surface area. This isconfirmed by measurements of the double layer capacitance chargingcurrents which correlate with the electrochemically accessible surfacearea. The double layer charging currents were measured by recordingcyclic voltammograms at 10 mv/sec in an electrolyte containing only 0.5M K₂ SO₄ in water. One half of the total current difference between thecathodic and anodic sweeps of the cyclic voltammogram measured at 0.0 Vvs Ag/AgCl was taken as the double layer charging current (Idl). As seenfrom the data in Table 13, double layer charging current, normalized forelectrode area and therefore fibril mass, is nearly constant even thoughthe carbon fiber content of the electrode materials varies over a widerange.

                  TABLE 13                                                        ______________________________________                                        Electrode Idl, mA       Area   Idl/Area                                       ______________________________________                                        184-28-1  0.0075        0.146  0.514                                          184-28-3  0.0085        0.182  0.466                                          184-28-4  0.100         0.175  0.572                                          ______________________________________                                    

EXAMPLE VIII(Inventive) Use of Fibril Mats as Flow Through Electrodes

A 50 ml suspension of carbon nanotubes in water was prepared at aconcentration of 1 milligram per milliliter. The suspension wassubjected to sonication with a 450 watt Branson probe sonicator at fullpower with a 20% duty cycle for 20 minutes to insure that the carbonnanotubes were well dispersed. The dispersion was vacuum filtered onto a0.45 μm MSI nylon filter in a 47 mm Millipore membrane filter manifold.The carbon nanotubes formed a felt-like mat that was peeled off of thenylon membrane and dried for two hours at 80° C. between two pieces offilter paper under weight to maintain flatness. The thickness (orheight) of the dried carbon nanotube mat was 8 mils measured withcalipers. A 13 mm arch punch was used to prepare a 13 mm diameter diskof the carbon nanotubes mat.

A electrochemical flow cell was constructed from a 13 mm, plastic,Swinney type membrane filter holder by placing a 13 mm diameter disk ofgold mesh (400 mesh, Ladd Industries) on top of the membrane support andmaking electrical contact to the screen with a platinum wire, insulatedwith Teflon® heat shrink tubing, that was fed through the wall of thefilter holder for external connection as the working electrode of athree electrode potentiostat circuit. The gold mesh was fixed in placewith a minimal amount of epoxy around the outer edge. A strip of goldfoil was fashioned into a ring and placed in the bottom, down streamsection of the filter holder and connected with a platinum wire lead fedthrough the wall of the filter holder for external connection as thecounter electrode of the three electrode potentiostat circuit. A ring of0.5 mm diameter silver wire was electrochemically oxidized in 1M HCl,rinsed with water and placed in the top section of the filter holderwith the end of the wire led out through the wall for externalconnection as the reference electrode in the three electrodepotentiostat circuit.

The appropriate external contacts on the flow cell were connected to theworking, counter and reference leads of an EG&G PAR 273 potentiostat.The flow cell was connected to a Sage syringe pump with interchangeablesyringes for use of different solutions. Initial background measurementswere made in 0.5M K₂ SO₄ by examining the cyclic voltammetric responseof the gold mesh to both static and flow at 0.6 milliliters/minute. Thesolution was switched to one containing 2.5 mM potassium ferricyanide,2.5 mM potassium ferrocyanide, 10 mM KCl and 0.5M K₂ SO₄ in water andcyclic voltammograms were recorded in static and 0.4 mls/minute flowconditions. There was a 0.225 mA difference between the peak anodic andthe peak cathodic currents at a scan rate of 10 mv/second under static,no flow conditions. These control experiments determined that thereference electrode was stable under flowing conditions and establishedthe background current levels due to the gold mesh support.

The 13 mm carbon nanotube mat disk was placed on top of the gold mesh,followed by the gasket supplied with the filter holder and the topsection of the filter holder. The cell was flushed with a solutioncontaining 2.5 mM potassium ferricyanide, 2.5 mM potassium ferrocyanide,10 mM KCl and 0.5M K₂ SO₄ in water. Cyclic voltammograms forferri/ferrocyanide reduction and oxidation recorded under staticconditions exhibited peak currents nearly 50 times greater than observedfor the gold mesh alone. There was a 12 mA difference between the peakanodic and the peak cathodic currents at a scan rate of 10 mv/secondunder static, no flow conditions. Furthermore, the shape of the cyclicvoltammograms recorded at scan rates of 10, 20, and 40 mv/second wasconsistent with oxidation and reduction of ferri/ferrocyanide entrappedwithin the pore structure of the carbon nanotube mat electrode. Similarcyclic voltammetric shapes were recorded under flow conditions with apumping rate of 0.4 mls/minute. The results are shown in FIG. 8 which isa graphical representation of the current-voltage relationship for thematerials made according to Example 8 wherein the vertical axisrepresents current and the horizontal axis represents applied potentialat several potential scan rates.

The terms and expressions which have been employed are used as terms ofdescription and not of limitations, and there is no intention in the useof such terms or expressions of excluding any equivalents of thefeatures shown and described as portions thereof, its being recognizedthat various modifications are possible within the scope of theinvention.

I claim:
 1. A composition of matter comprising a porous packed bedhaving a plurality of nanofibers and a number of scaffold particulates,said packed bed having a fluid flow rate characteristic for watergreater than 0.5 ml/min/cm² at a pressure differential through thepacked bed of about 1 atm when said packed bed has a thickness of onemil.
 2. The composition as recited in claim 1, wherein said packed bedhas a surface area greater than about 10 m² /g.
 3. The composition asrecited in claim 1, wherein said nanofibers have an average diameterless than 0.1 micron and said scaffold particulates have a firstdimension greater than about 1 micron and a second dimension greaterthan about 0.5 micron.
 4. The composition as recited in claim 1, whereinsaid packed bed has at least one fluid flow characteristic at least twotimes greater than a nanofiber packed bed without said scaffoldparticulates.
 5. The composition as recited in claim 1, wherein saidpacked bed comprises from 1 wt % to 99 wt % nanofibers and 99 wt % to 1wt % scaffold particulates.
 6. The composition as recited in claim 1,wherein said packed bed comprises from 5 wt % to 50 wt % nanofiber and90 wt % to 60 wt % scaffold particulate.
 7. The composition as recitedin claim 1, wherein said packed bed comprises a non-void solid volumehaving from 1 vol % to 99 vol % nanofibers and 99 vol % to 1 vol %scaffold particulates.
 8. The composition as recited in claim 1, whereinsaid packed bed comprises a non-void solid volume having from 5 vol % to50 vol % nanofiber and 90 vol % to 60 vol % scaffold particulate.
 9. Thecomposition as recited in claim 1, wherein said nanofibers have adiameter less than about 0.5 microns and a length to diameter ratiogreater than about
 5. 10. The composition as recited in claim 1, whereinsaid nanofibers are carbon fibrils being substantially cylindrical witha substantially constant diameter of less than 0.5 microns, havinggraphitic layers concentric with the fibril axis and being substantiallyfree of pyrolytically deposited carbon.
 11. The composition as recitedin claim 1, wherein said scaffold particulates are of a shape and a sizesuitable for providing a scaffolding effect within said packed bed. 12.The composition as recited in claim 1, wherein said scaffoldparticulates have a particulate shape selected from a fiber, irregularsolid, sphere, platelet, disc, pyramid or cube.
 13. The composition ofmatter as recited in claim 1, wherein said scaffold particulates arefibers having an average diameter at least 10 times greater than theaverage nanofiber diameter.
 14. The composition as recited in claim 1,wherein said nanofibers and said scaffold particulates comprise carbon.15. The composition as recited in claim 1, wherein said nanofibers arecarbon nanofibers and said scaffold particulates are carbon fibers. 16.The composition as recited in claim 15, wherein said particle additivesare present in an amount ranging from 0.01 to 25 wt%.
 17. Thecomposition as recited in claim 1, wherein said packed bed hassubstantially isotropic physical properties in at least two dimensions.18. The composition as recited in claim 1, further comprising particleadditives dispersed within the packed bed.